Evaluation method and fabrication method of optical element having multilayer film, exposure apparatus having the multilayer film, and device fabrication method

ABSTRACT

A fabrication method of an optical element having a multilayer film includes the steps of forming a multilayer film on a substrate, measuring a secondary radiation radiated from the multilayer film when a light with a wavelength of 2 to 40 nm is irradiated to the multilayer film, determining a phase difference between the light irradiated to the multilayer film and the light reflected from the multilayer film based on a measurement result of the measuring step, and modifying the multilayer film based on the determined phase difference.

CROSS REFERENCE TO RELATED APPLICATION

This is a divisional of and claims priority from U.S. patent applicationSer. No. 11/124,000 filed May 6, 2005, the content of which isincorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates generally to a fabrication method of anoptical element, and more particularly to an evaluation and fabricationmethod of an optical element having a multilayer film (for example, amultilayer mirror and reflection-type mask). Moreover, the presentinvention relates to an exposure apparatus having the optical elementand a device fabrication method using the exposure apparatus.

Reduction projection exposures using ultraviolet have beenconventionally employed to manufacture such a fine semiconductor deviceas a semiconductor memory and a logic circuit in lithography technology.However, the lithography using the ultraviolet light has the limit tosatisfy the rapidly promoting fine processing of a semiconductor device.Therefore, an exposure apparatus using extreme ultraviolet (“EUV”) lightwith a wavelength of approximately 13.5 nm shorter than that of theultraviolet (referred to as an “EUV exposure apparatus” hereinafter) hasbeen developed to efficiently transfer very fine circuit patterns of 50nm or less.

The EUV exposure apparatus uses a reflection-type optical element suchas a mirror for an optical system, and a multilayer film thatalternately forms or layers two kinds of materials having differentoptical constants is formed on a surface of the reflection-type opticalelement. The multilayer film is formed by alternately forming orlayering molybdenum (Mo) layer and silicon (Si) layer on a preciselypolished glass plate. The thickness of the layer is decided according tothe wavelength of the exposure light etc., and for example, a Mo layeris about 3 nm thickness, and a Si layer is about 4 nm thickness. A sumof the thicknesses of two kinds of materials is generally called acoating cycle, which is 7 nm in the above example.

The multilayer mirror reflects EUV light with a specific wavelength whenreceiving EUV light. Efficiency reflected EUV light is one within anarrow bandwidth around λ that satisfies an interference condition whereλ is a wavelength of the reflected EUV light, θ is an incident angle andd is a coating cycle and the bandwidth is about 0.6 to 1 nm. Theinterference condition is approximately expressible by Bragg's equation(Equation 1), but it shifts minutely from a value obtained from thisequation solely due to an influence of refraction in the material etc.2×d×cos θ=λ  (1)

The multilayer mirror in the projection optical system requires veryhigh precision for its surface shape. For example, a permissible figureerror σ (rms value) is given in Marechal's equation (Equation 2) belowwhere n is the number of multilayer mirrors in the projection opticalsystem, and λ is a wavelength of the reflected EUV light.

$\begin{matrix}{\sigma = \frac{\lambda}{28 \times \sqrt{n}}} & (2)\end{matrix}$

For example, six multilayer mirrors in the projection optical systemthat uses the exposure light with a wavelength of 13 nm is permitted tohave a figure error σ of 0.19 nm. The wave front aberration amount isabout 0.4 nm for resolution of 30 nm pattern transfer, which ispermitted for the whole projection optical system.

Therefore, a surface precision required for the multilayer mirror in theprojection optical system is very high, and the surface precision of 0.2nm as a phase is necessary.

The conventional fabrication method of the multilayer mirror includes aforming process of the multilayer mirror and a shape measuring process.

The multilayer mirror forming process polishes the substrate whilerepeating the shape measurement with the interferometer that usesvisible light, and forms a predetermined shape substrate. Next, themultilayer film is formed on the substrate surface. When actuallyfunctioning as the optical system, a best thickness distribution isformed in consideration of the angle and the wavelength of the lightirradiated to each position of the multilayer film on the mirrorsurface.

The shape measuring process measures the surface shape of the multilayermirror that completes the forming the multilayer film by theinterferometer that uses visible light, and judges whether the surfaceshape of the multilayer film satisfies the predetermined shape (in otherwords, above figure error σ). The multilayer mirror judged not to havethe predetermined surface shape exfoliates the multilayer because theforming the multilayer film has failed, and re-forms the multilayerfilm.

The method using a Point Diffraction Interferometer (PDI) that directlymeasures the reflection surface of the multilayer mirror by using theEUV light is known as other prior arts (see, for example, U.S. PatentApplication Publication No. 2002/044287 and Japanese Patent ApplicationPublication No. 2000-97620).

Moreover, the method that acquires the layer structure of the X-raymultilayer mirror and the information of interface roughness from theform of X-ray standing wave spectrum is known (see, for example,Japanese Patent Application Publication No. 2002-243669 and JapanesePatent Application Publication No. 2000-55841).

The data concerning the electronic energy loss in the material has beendisclosed (see, for example, Youta Nakai et al., “Stopping power of thematerial to electron of 10 keV or less”, Applied physics volume 51section 3, page 279, March, 1982). The model calculation concerning therelationship between the reflectivity of the multilayer film and thephase of the reflected light has been disclosed (see, for example, J. H.Underwood and T. W. Barbee, “Layered Synthetic Microstructures as BraggDiffractors for X-Rays and Extreme Ultraviolet: Theory and PredictedPerformance”, Applied Optics 20, 3027 (1981)). Moreover, thephotoelectric effect of the multilayer film has been disclosed (see, forexample, Michael E. Malinowski, Chip Steinhaus, W. Miles Clift, LeonardE. Klebanoff, Stanley Mrowka, Regina Soufli, “Controlling contaminationin Mo/Si multilayer mirrors by Si surface capping modifications”, Proc.SPIE Vol. 4688, Page 442-453, July 2002).

The conventional method measures the surface shape of the multilayermirror, can not obtain the wave front of the reflected light if thephase is not considered. Therefore, the wave front of the reflectedlight can not be correctly obtained, and it is difficult to accuratelycorrect the multilayer film.

The PDI method directly measures the reflection surface of themultilayer mirror, and the manufacturing of the apparatus is difficultbecause the size of the pinhole used to generate an accurate sphericalwave is very minute (for example, plural tens nm). Therefore, it isnecessary to use very high-luminance light source to introduce enoughamount of the EUV light into the minute pinhole, so the measurementsystem becomes very large and expensive.

Accordingly, it is an exemplary object of the present invention toprovide a fabrication method of an optical element that can easilyfabricate an optical element that has a multilayer film with a desiredperformance.

BRIEF SUMMARY OF THE INVENTION

A fabrication method of an optical element having a multilayer filmaccording to one aspect of the present invention includes the steps offorming a multilayer film on a substrate, measuring a secondaryradiation radiated from the multilayer film when a light with awavelength of 2 to 40 nm is irradiated to the multilayer film,determining a phase difference between the light irradiated to themultilayer film and the light reflected from the multilayer film basedon a measurement result of the measuring step, and modifying themultilayer film based on the determined phase difference.

A modification apparatus of an optical element having a multilayer filmaccording to another aspect of the present invention includes airradiating optical system for irradiating the multilayer film using alight with a wavelength of 2 to 40 nm, a detector for detecting asecondary radiation radiated from the multilayer film by irradiating thelight to the multilayer film, a modification part for modifying themultilayer film, and a controller for determining a phase differencebetween the light irradiated to the multilayer film and the lightreflected from the multilayer film based on a detection value of thedetector, and controlling the correction part based on the determinedvalue.

An evaluation method of an optical element having a multilayer filmaccording to another aspect of the present invention includes the stepsof measuring a secondary radiation radiated from the multilayer filmwhen a light with a wavelength of 2 to 40 nm is irradiated to theoptical element, determining a phase difference between the lightirradiated to the multilayer film and the light reflected from themultilayer film based on a measurement result, and judging whether touse the optical element based on the determined phase difference.

An exposure apparatus according to another aspect of the presentinvention includes a projection optical system for projecting a patternof a reticle onto an object, the projection optical system includes anoptical element having a multilayer film fabricated by a fabricationmethod, wherein the fabrication method includes the steps of forming themultilayer film on a substrate, measuring a secondary radiation radiatedfrom the multilayer film when a light with a wavelength of 2 to 40 nm isirradiated to the multilayer film, determining a phase differencebetween the light irradiated to the multilayer film and the lightreflected from the multilayer film based on a measurement result of themeasuring step, and modifying the multilayer film based on thedetermined phase difference.

A device fabrication method according to another aspect of the presentinvention includes the steps of exposing an object using an exposureapparatus, and performing a development process for the object exposed,wherein said exposure apparatus includes a projection optical system forprojecting a pattern of a reticle onto the object, the projectionoptical system includes an optical element having a multilayer filmfabricated by a fabrication method, wherein the fabrication methodincludes the steps of forming the multilayer film on a substrate,measuring a secondary radiation radiated from the multilayer film when alight with a wavelength of 2 to 40 nm is irradiated to the multilayerfilm, determining a phase difference between the light irradiated to themultilayer film and the light reflected from the multilayer film basedon a measurement result of the measuring step, and modifying themultilayer film based on the determined phase difference.

Other objects and further features of the present invention will becomereadily apparent from the following description of the preferredembodiments with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an equivalently shape change by a phasedifference and a phase change between an incident light and reflectedlight of a multilayer mirror as one embodiment according to the presentinvention.

FIG. 2 is a schematic view of a EUV exposure apparatus of the instantembodiment.

FIG. 3 is a flowchart for explaining a fabrication method of an opticalelement as one embodiment according to the present invention.

FIG. 4 is a typical view for measuring a field strength ratio of amultilayer film surface used for the instant embodiment.

FIG. 5 is a typical view for measuring an electric field strength ratioof a multilayer film surface used for the instant embodiment.

FIG. 6 is a graph of an incidence angle dependency of a reflectivity andan electric field strength ratio.

FIG. 7 is a schematic sectional view for explaining a measuring methodof a reflection surface shape of a multilayer mirror.

FIG. 8 is a graph of a measurement result of a wavelength dependency ofa reflectivity and an electric field strength ratio obtained by thefirst embodiment.

FIG. 9 is a graph of a wavelength dependency of a phase differencebetween an incident light and reflected light obtained by the firstembodiment.

FIG. 10 is a schematic sectional view of one example of a multilayerstructure used for the first embodiment.

FIG. 11 is a schematic sectional view of another example of a multilayerstructure used for the first embodiment.

FIG. 12 is a graph of a measurement result of a wavelength dependency ofa reflectivity and a photoelectron discharge amount used for the secondembodiment.

FIG. 13 is a graph of a relationship among a film thickness, wavelength,electric field strength, and reflectivity by model calculation used forthe second embodiment.

FIG. 14 is a graph of a relationship among a film thickness, wavelength,phase, and reflectivity by model calculation used for the secondembodiment.

FIG. 15 is a graph of a relationship among an incidence angle,reflectivity, and electric field strength by model calculation used forthe third embodiment.

FIG. 16 is a block diagram of an equivalently shape change by a phasedifference and a phase change between an incident light and reflectedlight of a multilayer mirror as another embodiment according to thepresent invention.

FIG. 17 is a view of a reflected light as which phase is the same.

FIG. 18 is a comparison view of a wave front of a reflected light at apart where a couple of layer is different.

FIG. 19 is a graph of a relationship between a number of a coating cycleof a multilayer film and a reflectivity standardized by the maximumvalue.

FIG. 20 is a graph of a relationship among a reflectivity, shift amountof a wave front or value in which the shift amount of the wave front isconverted into a shift of spatial reflection position, and millingdepth.

FIG. 21 is a view for explaining correcting method of wave frontaberration that originates in a figure error of a mirror substrate.

FIG. 22 is a view for explaining correcting method of wave frontaberration that originates in a figure error of a mirror substrate.

FIG. 23 is a view of a multilayer mirror to which a deformation is addedby a piezo electric element.

FIG. 24 is a flowchart for explaining how to fabricate devices (such assemiconductor chips such as ICs, LCDs, CCDs, and the like).

FIG. 25 is a detailed flowchart of a wafer process in Step 4 of FIG. 24.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIG. 3, a description will be given of a main part of afabrication method of an optical element having a multilayer film of theinstant embodiment. Here, FIG. 3 is a flowchart of the main part of thefabrication method of the optical element of the instant embodiment.

First, a substrate is polished (step 1001). Next, a surface shape ismeasured by visible light (step 1002). Next, a multilayer film that hasa predetermined film thickness on the substrate of predetermined shapeis formed (step 1003).

Next, the surface shape of the multilayer film is measured during areflection position measurement step (step 1004 a) and a visible wavefront measurement step (step 1005 a). In this case, the conventionalshape measuring process measures only a geometrical shape of themultilayer surface. On the other hand, the instant embodimentcalculates, as described later, an equivalent shape of the reflectionsurface based on a phase difference between an incident light and areflected light in addition to the geometrical shape of the multilayersurface. The phase difference between the incident light and thereflected light is calculated by using the standing wave generated whenlight with a wavelength of 2 to 40 nm (EUV light) is irradiated to themultilayer film of optical element (for example, multilayer mirror orreflection-type mask) that the multilayer film that has thepredetermined film thickness on the substrate of predetermined shape isformed, and a shape of “equivalent reflection surface” described lateris calculated by using the phase difference.

Next, the EUV reflection wave front is calculated (step 1006 a) to judgewhether a design value of the shape of “equivalent reflection surface”or a difference (error) from an ideal shape is within a tolerance (forexample, above figure error σ).

In step 1006 a, if multilayer film is judged to not be within thetolerance, the multilayer film forming is improper, and the multilayerfilm (step 1010) is corrected (modified). This correcting method(modifying method) is a method of using a coating milling shown by thefollowing embodiment, a method of adding an addition film, and a methodof applying heat etc. The trouble of the multilayer film is corrected byvarious correcting methods, and it returns to steps 1004 b and 1005 b. Aregulated frequency (for example, three times) repeats this correcting,measuring, and calculating, the multilayer film is exfoliated for NG(step 1011), and a new multilayer film is formed to the substrate. Afterthe multilayer film forms, the multilayer film is measured by thereflection position measurement (step 1004 a) and the visible wave frontmeasurement (step 1005 a), and it judges that it is within thetolerance.

A regulated frequency (for example, three times) repeats theexfoliating, forming, and measuring steps, and the regulated frequencyis repeated for the polishing a substrate recycle for NG. A problem iscreated when the multilayer film is not within the tolerance. Then, whenthere is the problem in the substrate, it is judged that the substraterecycle is impossible, and abandons the substrate (step 1009).

On the other hand, in step 1006 a, if it is judged that it is within thetolerance, the forming the multilayer film considers suitableness. Then,a necessary amount of correction optimal value is calculated when theoptical element is built into a lens barrel (step 1007), and it advancesto a built-in to the lens barrel process (step 1008).

The instant embodiment measures the shape of equivalent reflectionsurface that originates in the phase difference as shown in steps 1004to 1006, and improves the shape measurement accuracy. Then, the instantembodiment calculates the wave front of the reflected light from themultilayer film, and facilitates the correction of the multilayer film.

A description will be given of the calculation of the shape of“equivalent reflection surface”.

The wave front of light is defined as surface where the phase of anelectromagnetic field vibration is equal, and is orthogonal for thelight beam shown geometrical. The wave front of parallel light is aplane surface that is orthogonal for the traveling direction of light,and such light is called a plane wave.

For a simple example, there is the case where the plane wave ofincidence angle 0° reflects from a plane mirror. The wave front is aparallel plane to the mirror surface because of the incidence angle 0°.When a phase difference by reflection on the mirror surface, in otherwords, the phase difference between the reflected light and the incidentlight is constant everywhere on the mirror surface, the incident lightreceives a phase change in a constant amount by the reflection.Therefore, a wave front of the reflected light (=equivalent phasesurface) is also parallel plane to the mirror surface.

Next, there is the case where the plane wave reflects from a mirror thatis not plane surface. If the phase difference by the reflection on themirror surface is constant everywhere on the mirror surface, thereflected light receives a constant phase change. However, because anoptical path difference is caused by a convexoconcavity of the mirrorsurface, the wave front of the reflected light (=equivalent phasesurface) shifts from the plane surface. When the wave front has risen hat a single position on the mirror surface, the wave front of thereflected light becomes a shape (away from the mirror) that 2h rises atthe position corresponding to it.

When the phase difference generated by the reflection on the mirrorsurface is partially different than the mirror surface even if a planemirror, the reflected light receives the phase change that is differentaccording to place. Therefore, a wave front of the reflected light(=equivalent phase surface) becomes a shape that shifts from the planesurface. When the phase difference of the place on the plane mirror bythe reflection is δ (rad) larger than the surrounding area, the wavefront of the reflected light reflected from the place becomes a shape(away from the mirror) that rises δλ/2π from the mirror surface comparedwith the wave front that reflected from the surrounding area. Here, λ isthe wavelength of the incident light. In this case, the phase differenceby the reflection on the surface is constant, and is equivalent to thecase for reflecting at a mirror that the mirror surface rises δλ/2π.

Thus, a mirror shape to convert the phase difference by the reflectionon the surface into the mirror shape is called “equivalent reflectionsurface”.

Moreover, there is a case that the wave front is reflected by a mirrorwhere the surface is not a plane surface and the phase differencegenerated by the reflection is not constant in the surface. When theshape at the position of the mirror rises h, in addition, the phasedifference of this position by the reflection is δ larger than thesurrounding area, it becomes a combination of above 2 examples.Therefore, the shape of the reflected light becomes a shape (away fromthe mirror) that rises 2h+δλ/2π. In this case, the equivalentlyreflection surface becomes a shape that rises (h+δλ/4π).

When the incidence angle θ is not 0°, a similar idea consists if ageometrical optical path difference is corrected, and in general,equivalently reflection surface is corrected to h+δλ/(4π cos θ). Theconventional shape measuring process measures only h, but the instantembodiment calculates h+δλ/(4π cos θ), and compares this with the figureerror σ. Moreover, the conventional can not accurately measure the phasedifference δ, but the instant embodiment can measure the phasedifference δ easily and accurately as explained as follows. The instantembodiment explained that the plane wave is incident for thesimplification. When the incident light is not the plane wave but aspherical wave or a case of superimposing the aberration, it is possibleto approximate to the plane wave if there is a enough small area.

When a monochromatic parallel EUV light is incident to the multilayerfilm, the EUV light reflected from this multilayer film has the samephase difference as the incident light. The incident light and thereflected light interfere inside and outside of the multilayer film, andthe standing wave is generated. The present invention obtains the abovephase difference δ by using the standing wave, and accurately measures arelationship between the wave front of the incident light and the wavefront of the reflected light. The method in detail is explained asfollows.

When the EUV light is incident to the multilayer film and the EUV lightreflects from multilayer film, the phase difference between the incidentlight and the reflected light changes due to a multilayer filmstructure, an optical constant of material that composes the multilayerfilm, an incident angle, and the wavelength of the EUV light, etc. Anelectric field amplitude of the reflected light is r×E₀, where E₀ is anelectric field strength of the incidence EUV light, and r is anamplitude reflectance of the incidence EUV light. The phase difference δbetween the incident light and the reflected light, and an amplitude Eof the electric field where the incident light is overlapped with thereflected light is shown by Equation 3.E−E₀(1+r×cos δ)  (3)

The electric field strength is proportional to the square of theamplitude, and the reflectivity R of light is the square of theamplitude reflectance r. Thereby, a ratio (electric field strengthratio) of an electric field strength I (of the standing wave generatedby interference of the incident light and the reflected light) of themultilayer film and an electric field strength I₀ of the incident lightis shown by Equation 4.I/I ₀=(1+R+2×R ^(1/2)×cos δ)  (4)

The ratio I/I₀ of the electric field strength of the multilayer filmsurface and the electric field strength of the incident light and thereflectivity R are calculated from Equation 4, and the phase differenceδ can be obtained. The reflectivity R can be easily measured bymeasuring the light intensity of the incident light and the lightintensity of the reflected light and calculating a ratio of both. Thisresult of a measuring method of the ratio I/I₀ of the electric fieldstrength of the multilayer film surface and the electric field strengthof the incident light is explained in detail as follows.

When the EUV light is irradiated to the material in the vacuum, a partof light is absorbed to the material, and a photoelectric effect iscaused, and an electron is discharged. An amount of the dischargedphotoelectron is proportional to electric field strength at theposition. Then, a photoelectric detector such as a microchannel plateand an electron multiplier is installed near an irradiation area of theEUV light on the multilayer film shown in FIG. 4, and the amount of thephotoelectron is measured.

When the photoelectric effect occurs on the surface of the material, thedischarged electron is discharged with little energy loss in the vacuum.This phenomenon is an external photoelectric effect. On the other hand,when the a photoelectric effect occurs in the material (a position fromthe surface that is deeper than about 1 nm), the discharged electroninelastically collides with a surrounding atom, loses energy rapidly,and little energy is discharged in the vacuum. The majority of theenergy is lost when discharged from the atom even if discharged in thevacuum, and therefore the electron with low energy is discharged.Therefore, the amount of the electron discharged in the vacuum by theexternal photoelectric effect is proportional to the electric fieldstrength in the nearest surface of the material (an area from thesurface that is shallower than about 1 nm). When the EUV light with aincidence angle of θ₀ that satisfies the Bragg's condition (law) and awavelength of λ is incident to the multilayer film composed to obtainthe high reflectivity by the predetermined incidence angle of θ₀ and thepredetermined wavelength of λ₀, an amount Q_(R) of the photoelectrondischarged from the surface in the vacuum is proportional to theelectric field strength of the standing wave of the nearest surface ofthe material generated by the interference of the incident light and thereflected light.

When the EUV light is irradiated to the material in the vacuum, a partof light is absorbed to the material, for example, not only thephotoelectron but also, for example, fluorescent X-ray as othersecondary radiations is discharged. An amount of the dischargedfluorescent X-ray is proportional to electric field strength at theposition. Then, the amount of fluorescent X-ray is measured by an X-raydetector as the detector shown in FIG. 4, and the amount of fluorescentX-ray may be used as above Q_(R).

The energy of fluorescent X-ray has a peculiar energy to the atom thatdischarges it. Therefore, the electric field strength at the position ofa specific atom can be measured by being spectrum as for fluorescentX-ray and measuring only the light intensity of X-ray that has thepeculiar energy.

Therefore, a thin film composed of an element different from an elementthat composes the multilayer film is formed on the surface of themultilayer film, and if the light intensity of the peculiar fluorescentX-ray reflected from this element is measured, the electric fieldstrength of the near surface of the multilayer film can be measured.

When the EUV light with a greatly different incidence angle from theincidence angle θ₀ and a wavelength of λ₀ is incident to this multilayerfilm, the reflectivity remarkably decreases because it deviates from thecondition of strengthening the light intensity of reflected light, andthe light intensity of the reflected light becomes remarkably smallcompared with the light intensity of the incident light. At this time,an amount Q₀ of the photoelectron discharged from the multilayer filmsurface in the vacuum is almost proportional to the electric fieldstrength of the incident light. At this time, when the incidence angleis near 90 degrees, the reflectivity rises by total reflection'soccurring. Then, the incidence angle may be not near 90 degrees.

FIG. 6 is a graph of an example of the incidence angle dependency of thereflectivity and the electric field strength ratio. The electric fieldstrength ratio is a ratio between the electric field strength of themultilayer film surface and the electric field strength of the incidentlight. This example is wavelength 13.5 nm. A multilayer film thatoptimized so that the reflectivity may become a peak in the incidenceangle 10° is used. In this example, the reflectivity is, in range ofabout 20 degrees to 70 degrees of the incidence angle, 1/10 or less ofthe peak reflectivity (about 70%) in the incidence angle 10°, andstandardized electric field strength is a value close to 1. In otherwords, in range of about 20 degrees to 70 degrees of the incidenceangle, the electric field strength on the multilayer film surface isalmost equal to the electric field strength of the incident light. Theamount Q₀ of the photoelectron discharged from the multilayer filmsurface in the vacuum at this time is almost proportional to theelectric field strength of the incident light in such a range of theangle degrees. The amount Q₀ of the photoelectron discharged from thesurface of the multilayer film, at an angle greatly different from theincidence angle where the reflectivity is maximum value, and to whichthe reflectivity decreases in the vacuum is almost proportional toelectric field strength of the incident light for the multilayer filmthat the reflectivity becomes a peak in an incidence angle differentfrom the example of the instant embodiment.

Then, the EUV light is irradiated on the multilayer film by two angleconditions with first condition that the incidence angle to obtain highreflectivity by satisfying Bragg's condition and second condition thatthe incidence angle to which the reflectivity remarkably decreasescompared with the first condition without satisfying Bragg's condition,and the amounts Q_(R) and Q₀ of the photoelectron discharged in thevacuum are obtained. Thereby, a ratio of the electric field strength Iof the multilayer film (standing wave) and the electric field strengthI₀ of the incident light can be obtained by the following Equation 5.Q _(R) /Q ₀ =I/I ₀  (5)

When there is a possibility that the light intensity of the incidentlight changes between two measurement conditions with differentincidence angle, a detector that measures the light intensity of theincident light is installed, the amount of the electron discharged inthe vacuum is standardized by the light intensity of incident light, anerror because of the change of the light intensity of the incident lightcan be suppressed. When the EUV light with the incidence angle of θ₀that satisfies the Bragg's condition (law) and the wavelength of λ isirradiated to the multilayer film composed to obtain the highreflectivity by the predetermined incidence angle of θ₀ and thepredetermined wavelength of λ₀, the amount of the photoelectrondischarged from the surface in the vacuum is measured by the measuringapparatus shown in FIG. 1. At this time, a light intensity measured by alight intensity monitor 14 is assumed to be I_(OR).

When the EUV light with a greatly different incidence angle from theincidence angle θ₀ and a wavelength of λ₀ is incident to this multilayerfilm, the reflectivity remarkably decreases because it deviates from thecondition of strengthening the light intensity of reflected light, andthe light intensity of the reflected light becomes remarkably smallcompared with the light intensity of the incident light. At this time,when the amount Q₀ of the photoelectron discharged from the multilayerfilm surface in the vacuum is measured, the beam strength measured bythe light intensity monitor 14 is assumed to be I₀₀. The error becauseof the change of the light intensity of the incident light can besuppressed by standardizing the amount of the photoelectron dischargedin the vacuum by the light intensity of the incident light (see,Equation 6).(Q _(R) ×I ₀₀)/(Q ₀ ×I _(0R))=I/I ₀  (6)

When the EUV light with a wavelength of λ that shifts from thewavelength of λ₀ in which Bragg's condition is satisfied and theincidence angle θ₀ is irradiated to the multilayer film that satisfiesBragg's condition, the reflectivity remarkably decreases because itdeviates from the condition of strengthening the light intensity ofreflected light, and the light intensity of the reflected light becomesremarkably small compared with the light intensity of the incidentlight. For example, in FIG. 8, the reflectivity is 1/10 or less, and avery small value compared with the peak reflectivity in a wavelengthband that deviates from wavelength of 12.8 to 14 nm.

At this time, an amount Q_(L) of the photoelectron discharged from themultilayer film surface in the vacuum is almost proportional to theelectric field strength of the incident light. However, if wavelengthused is greatly differs from the wavelength of λ₀ in which Bragg'scondition is satisfied, a discharge efficiency of the photoelectron(discharged photoelectric number by incidence photon unit) shifts, and awavelength close to λ₀ is used. Concretely, the discharge efficiency ofthe photoelectron changes rapidly on the boundary of the absorption edgewavelength of the element that composes the multilayer film surface, andthe wavelength may be changed within the range where the absorption edgewavelength of the element that composes the multilayer film surface isnot exceeded.

The EUV light is irradiated to the multilayer film while changing thewavelength, and amounts Q_(R) and Q_(L) of the photoelectron dischargedin the vacuum in two wavelengths with first wavelength to obtain highreflectivity and second wavelength to which reflectivity remarkablydecreases compared with it are obtained. Then, a ratio of the electricfield strength I of the multilayer film surface and the electric fieldstrength I₀ of the incident light can be obtained from Equation 7.Q _(R) /Q _(L) =I/I ₀  (7)

As similar to the case measured at the different angle, when there is apossibility that the light intensity of the incident light changesbetween two measurement conditions with different wavelength, a detectorthat measures the light intensity of the incident light is installed,the amount of the photoelectron discharged in the vacuum is standardizedby the light intensity of incident light, an error because of the changeof the light intensity of the incident light can be suppressed.

When the EUV light is incident to a single layer film that is composedof a same material that composes the multilayer film surface, thereflectivity becomes extremely small, and the light intensity of thereflected light becomes very small compared with the light intensity ofthe incident light. At this time, an amount Q₀₀ of the photoelectrondischarged from the single layer film surface in the vacuum is almostproportional to the electric field strength of the incident light.

Therefore, an amount Q_(R) of photoelectron discharged in vacuum whenthe EUV light with the incidence angle to obtain high reflectivity isirradiated to the multilayer film and an amount Q of photoelectrondischarged in vacuum when the EUV light is irradiated to the singlelayer film that is composed of the same material that composes themultilayer film surface can be obtained, and a ratio of the electricfield strength I of the multilayer film surface and the electric fieldstrength I₀ of the incident light can be obtained from Equation 8.Q _(R) /Q ₀₀ =I/I ₀  (8)

In a similar case, when there is a possibility that the light intensityof the incident light changes between two measurement conditions, adetector that measures the light intensity of the incident light isinstalled, the amount of the photoelectron discharged in the vacuum isstandardized by the light intensity of incident light, an error becauseof the change of the light intensity of the incident light can besuppressed.

Next, a phase δ is calculated by the following Equation 9.cos δ=(I/I ₀−1−R)/(2×R ^(1/2))  (9)

When the phase difference is obtained from the cosine of the phasedifference, there is an uncertainty of integral multiples of 2π in thephase difference, but the phase difference may be continuous in thecontinuously measured area or for the wavelength change. There is anuncertainty of the positive-negative of the phase difference, but it mayhave a positive inclination in the near reflection peak of themultilayer film.

The phase difference δ between the phase of the incident light and thephase of the reflected light can be obtained by measuring the ratio I/I₀of the electric field strength of the multilayer film surface and theelectric field strength of the incident light by using the above method.Next, a description will be given of a method of obtaining the wavefront of the EUV light reflected by the multilayer film.

The surface shape of the multilayer film (in other words, above h) canbe measure with high accuracy by an already-known method in art, forexample, a method that mechanically directly measures the shape bytouching the stylus to the surface, and a method using an interferometerthat uses the visible light and ultraviolet light, etc.

When the phase difference δ between the incident light and the reflectedlight by reflection on the mirror surface is constant in the mirrorsurface and does not depend on the incidence angle, the wave front ofthe EUV light reflected by the multilayer film can be obtained by usingan usually ray tracing method and diffraction integration method, etc.(see, for example, Kunio Tsuruta, Applied optics I, (July, 1990 issue)).

When the phase difference δ between the incident light and the reflectedlight by reflection on the mirror surface is changed in the mirrorsurface and depends on the incidence angle, the wave front of the EUVlight reflected by the multilayer film can be obtained from themultilayer film shape by adding the optical path length of only δλ/2π onthe multilayer film surface and using the diffraction integrationmethod, etc.

The equivalently surface shape is shown by Equation 10, where acoordinate on the mirror surface is x and y, a geometrical surface shapeof the multilayer mirror is h (x, y), an inclination of a mirror normalto XY plane is φ (x, y), an incidence angle distribution of the EUVlight to the mirror surface is θ (x, y), and the phase differencebetween the incident light and the reflected light as the EUV light is δ(x, y, λ, θ). The wave front of the reflected light or the light beam isobtained by the ray tracing method by using this equivalently surfaceshape. Here, FIG. 7 is a schematic sectional view for explaining ameasuring method of the reflection surface shape of the multilayermirror.

$\begin{matrix}{{Z\left( {x,y,\theta} \right)} = \;{{h\left( {x,y} \right)} + \frac{{\lambda\delta}\left( {x,y,\lambda,\theta} \right)}{4\pi\;\cos\;{\theta\left( {x,y} \right)}\;\cos\;{\varphi\left( {x,y} \right)}}}} & (10)\end{matrix}$

Thus, the instant embodiment obtains the phase difference δ between theincident light and the reflected light by measuring the secondaryradiation discharged by the standing wave caused when the EUV light isincident to the multilayer film, and obtains h+δλ/(4π cos θ) as theequivalently surface shape to the EUV light or the wave front of the EUVlight reflected by the multilayer film from the measurement result ofthe geometrical surface shape of the multilayer surface (in other words,h) and the phase difference δ. The conventional shape measuring processobtains h, but the shape measuring process of the instant embodimentobtains h+δλ/(4π cos θ), and the accuracy of the shape measuring processimproves to the EUV light process. As a result, the instant embodimentcan accurately execute the correction of the multilayer film. Moreover,the instant embodiment can easily obtain the phase difference betweenthe incident light and the reflected light by the measuring apparatus bythe addition of the photoelectron or fluorescent X-ray detector to areflectivity measuring apparatus, and can be measured with high accuracyusing a very small apparatus compared with the conventional interferencemeasuring method such as PDI etc.

A description will be given of a more concretely embodiment.

First Embodiment

A description will be given of a more concrete method for obtaining“equivalent reflection surface” in the first embodiment.

FIG. 1 is a schematic block diagram of a measuring apparatus 1 of thefirst embodiment. The EUV light radiated from a EUV light source 10 as asynchrotron radiation light source, laser plasma light source, anddischarge plasma light source etc. is separated only to predeterminedwavelength by a spectroscope 12, and is made monochrome. The EUV lightmade monochrome is led to a multilayer mirror (or sample) ML as ameasurement target or a measuring room 20 that houses a detector 24 and26. The spectroscope 12 and the light intensity monitor 14 compose anirradiation system. The measuring room 20 has been exhausted to theultra-high vacuum by an exhaust part 21 such as a vacuum pump etc. toprevent an attenuation of the EUV light and diffusion of thephotoelectron, or a pollution adhering to the multilayer film surface.The multilayer mirror ML as the measurement target is fixed on a stage22 that can be rotated and moved in a direction of translation, and theEUV light made monochrome is incident to a predetermined position of themultilayer mirror ML at a predetermined angle. The EUV light reflectedby the multilayer mirror ML is led to a EUV light detector 24, and alight intensity of the reflected light is measured. The detector 24 usesa photodiode, photoelectron multiplier, and CCD, etc. The multilayermirror ML can be removed by the stage 22, and light intensity of theincident light is measured by directly irradiating the EUV light mademonochrome to the detector 24. The output of the detector 24 isconverted into a voltage signal by a charge sensitive amplifier, thevoltage signal is converted into a digital signal by using ananalog-digital converter (ADC) 18, and the digital signal is taken intoan operation part 16 such as computers. The operation part (controller)16 can obtains the reflectivity R by calculating the ratio of the lightintensity of the reflected light reflected by the multilayer mirror MLand the light intensity of the incident light.

The incident light intensity monitor 14, that measures the lightintensity of the monochrome EUV light led to the measuring room 20, hasbeen installed to correct a time change of the light intensity radiatedfrom the light source 10. When the synchrotron radiation light source isused, it may be assumed the incident light monitor by measuring thecurrent of an electron accumulation ring of the light source.

A detector 26 that detects the photoelectron is installed near themultilayer mirror. The detector uses an electron multiplier, and microchannel plate (MCP), etc. An incident electrode of the detector 26 isset to become positive potential for multilayer mirror ML so that thedischarged photoelectron is taken easily. When the photoelectrondischarged from the surface of the multilayer mirror ML is incident tothe electron multiplier or the MCP, the photoelectron receives theelectron doubling function by a high voltage that is applied inside, andthe photoelectron is output as an amplified electric charge signal. Thisis converted into the voltage signal using the charge sensitiveamplifier, the voltage signal is converted into the digital signal byusing the analog-digital converter (ADC) 18, and the digital signal istaken into the operation part 16.

The instant embodiment measures the phase of the reflected lightaccording to the following process.

First, the multilayer sample (multilayer mirror) ML is removed by thestage 22, and the light intensity of the incident light is measured bythe detector 24. At this time, wavelength is scanned while changing thewavelength λ of the EUV light that exits from the spectroscope 12, andthe wavelength dependency of the light intensity of the incident lightis measured. The light intensity of the incident light is I_(R0)(λ), andan output of the incident light intensity monitor at the measurement isI₀₀(λ).

Next, the EUV light made monochrome is set to be incident to thepredetermined position of the multilayer film mirror at thepredetermined angle by the stage 22, and the light intensity of thereflected light is measured by the detector. The amount of thephotoelectron discharged from the sample surface is measured by thedetector 26 at the same time. At this time, wavelength is scanned whilechanging the wavelength setting of the spectroscope 12, and thewavelength dependency of the light intensity of the reflected light andthe wavelength dependency of the amount of the photoelectron dischargedfrom surface of the sample ML are measured at the same time. The lightintensity reflected by the multilayer sample is I_(R1)(λ), the measureddischarged amount of the photoelectron of multilayer film is Q_(S)(λ),and the output of the incident light intensity monitor at themeasurement is I₀₁(λ).

Next, a discharged amount of a photoelectron of a single-layer mirrorcomposed of the material that composes the top layer of the multilayerfilm as reference sample RS is measured. A thickness of the single layerfilm of reference sample (or single-layer mirror) RS is more thick thanan escape depth of the photoelectron, and the transmittance of themeasured light is very small. If the wavelength of light is about 13.5nm, Mo, Si, and ruthenium (Ru), etc. have a thickness of plural hundredsnm or more. When the top layer of the multilayer film is Si, the Siwafer may be used.

A wavelength dependency of the amount of the photoelectron dischargedfrom the sample surface is measured for the reference sample RS by themethod similar to the multilayer film sample ML. The electric field onthe sample surface is obtained by adding the electric field of theincident light and the electric field of the reflected light. However,because the reflectivity of the single-layer mirror RS is very smallcompared with the EUV light, the electric field strength of the singlelayer film surface is almost equal to the electric field strength of theincident light. At this time, the wavelength dependency of the measureddischarged amount of the photoelectron of the reference sample isQ_(R)(λ), and the output of the incident light intensity monitor at themeasurement is I₀₂(λ).

The wavelength dependency R(λ) of the reflectivity of the multilayermirror ML is shown by Equation 11.R(λ)=(I _(R1)(λ)×I ₀₀(λ))/(I _(R0)(λ)×I ₀₁(λ))  (11)

The wavelength dependency F(λ) of the ratio of the discharged amount ofthe photoelectron of the multilayer mirror ML and the discharged amountof the photoelectron of the reference sample RS is shown by Equation 12.F(λ)=(Q _(S)(λ)×I ₀₂(λ))/(Q _(R)(λ)×I ₀₁(λ))  (12)

F(λ) is a parameter that shows how many times the discharged amount ofthe photoelectron of the multilayer mirror ML compared with thesingle-layer mirror RS. The electric field strength of the single-layerfilm surface is almost equal to the electric field strength of theincident light, and the ratio F(λ) of the discharged amount of thephotoelectron of the multilayer mirror and the discharged amount of thephotoelectron of the reference sample is equal to an amount (electricfield strength ratio) in which whether how many times the electric filedstrength of the multilayer film surface compared with the electric fieldstrength of the incident light are shown. FIG. 8 shows the measurementresult of the wavelength dependency of the reflectivity and the electricfield strength ratio.

Commutatively, the EUV light is irradiated to the multilayer film whilechanging the wavelength, and amounts Q_(R) and Q_(L) of thephotoelectron discharged in the vacuum in two wavelengths with the firstwavelength (of 13.5 nm in the instant embodiment) to obtain highreflectivity and second wavelength (of 12.5 or 14.5 nm in the instantembodiment) in which reflectivity remarkably decreases compared with itare obtained. A parameter that shows how many times the dischargedamount of the photoelectron of the multilayer mirror sample comparedwith the single-layer mirror is obtained by Equation 13, and thisparameter may be considered the electric field strength ratio.F=Q _(R) /Q _(L) =I/I ₀  (13)

Next, the phase δ is calculated by the following Equation 14.cos δ=(F(λ)−1−R(λ))/(2×R(λ)^(1/2))  (14)

When the phase difference δ is obtained from the cosine of the phasedifference δ, there is an uncertainty of integral multiples of 2π in thephase difference δ, but the phase difference δ may be constant in thecontinuously measured area for the wavelength change. There is anuncertainty of the positive-negative of the phase difference δ, but itmay have a positive inclination in the wavelength band of nearreflection peak of the multilayer film. FIG. 9 shows the wavelengthdependency of the phase difference δ between the incident light and thereflected light obtained thus.

The surface shape of the multilayer film sample is measured by a Fizeauinterferometer and Mirror interferometer, etc. that use visible light orultraviolet light. The surface shape measurement and the phasemeasurement by the standing wave are acceptable ahead either, and may besimultaneous.

Next, the shape of equivalent reflection surface for the EUV light (inother words, δλ/(4π cos θ) and h+δλ/(4π cos θ)) is calculated.

FIG. 10 shows one example of the multilayer film structure. This examplecauses a difference in the lowest layer of the multilayer film, and apart B is higher than a part A. The coating cycle is 6 nm, thewavelength of the incidence EUV light is 12 nm, the incidence angle is0°, and the difference of the part B is 1.5 nm. When the shape ismeasured by the interferometer that uses visible light, the part B ismeasured high of 1.5 nm compared with the part A. The phase differencebetween the part A and the part B is not measured by the above phasemeasurement that uses the standing wave. Therefore, the part B is 1.5 nmhigher than the part A in the shape of equivalently reflection surfacefor the EUV light. Thereby, when the plane wave is incident to thismultilayer film, the part B becomes a shape advanced only by 3 nm (about¼ wavelength) compared with the part A in the wave front of thereflected light.

FIG. 11 shows another example of the multilayer film structure. Thisexample causes a difference in the top layer of the multilayer film, anda part D is higher than a part C. The coating cycle is 6 nm, thewavelength of the incidence EUV light is 12 nm, the incidence angle is0°, and the difference of the part D is 1.5 nm. When the shape ismeasured by the interferometer that uses visible light, the part D ismeasured high of 1.5 nm compared with the part C. The phase differencebetween the part C and the part D is π/2 by the above phase measurementthat uses the standing wave. Therefore, the shape of equivalentreflection surface for the EUV light is denied by the surface shape andthe reflection phase difference each other, and becomes plane surface.In other words, when the plane wave is incident to the multilayer film,the wave front of the reflected light becomes plane surface.

Second Embodiment

A description will be given of other more concretely method forobtaining “equivalent reflection surface” in the second embodiment.

The instant embodiment uses a similar measuring apparatus 1 shown in thefirst embodiment. The instant embodiment measures the phase of thereflected light according to the following process. First, themultilayer mirror ML is removed by the stage 22, and the light intensityof the incident light is measured by the detector 24. At this time,wavelength is scanned while changing the wavelength λ of the EUV lightthat exits from the spectroscope 12, and the wavelength dependency ofthe light intensity of the incident light is measured. The lightintensity of the incident light is I_(R0)(λ), and an output of theincident light intensity monitor at the measurement is I₀₀(λ). Next, theEUV light made monochrome is set to be incident to the predeterminedposition of the multilayer mirror at the predetermined angle, and thelight intensity of the reflected light is measured by the detector. Theamount of the photoelectron discharged from the surface of the sample(multilayer mirror) ML is measured by the detector 26 at the same time.At this time, wavelength is scanned while changing the wavelengthsetting of the spectroscope 12, and the wavelength dependency of thelight intensity of the reflected light and the wavelength dependency ofthe amount of the photoelectron discharged from the surface of thesample ML are measured at the same time. The light intensity reflectedby the multilayer sample is I_(R1)(λ), the measured discharged amount ofthe photoelectron of multilayer film is Q_(S)(λ), and the output of theincident light intensity monitor at the measurement is I₀₁(λ). Asabove-mentioned, the wavelength dependency R(λ) of the reflectivity ofthe multilayer mirror ML is shown by Equation 11.

On the other hand, the wavelength dependency G(λ) of the dischargedamount of the photoelectron of the multilayer mirror ML is shown byEquation 14.G(λ)=Q _(S)(λ)/(I ₀₁(λ)×λ)  (14)

This is a parameter that shows a ratio between the discharged amount ofthe photoelectron of the multilayer mirror and a photon number of theincident light. The discharged amount of the photoelectron of one photonis almost constant in the wavelength bands other than in theneighborhood of the wavelength of absorption edge of the material thatcomposes the top layer of the multilayer film. Therefore, G(λ) is anamount (electric field strength ratio) in which whether how many timesthe electric field strength of the multilayer film surface compared withthe electric field strength of the incident light are shown. FIG. 12shows an example of the measurement result of the wavelength dependencyof the reflectivity and the discharged amount G of the photoelectron.

Next, the phase δ(λ) is calculated by the model calculation of themultilayer film. The reflectivity of the multilayer film and the phaseof the reflected light can be obtained by the model calculation. Forexample, the model calculation is disclosed in “Controllingcontamination in Mo/Si multilayer mirrors by Si surface cappingmodifications”.

Fresnel equations are applied to the each interface of the multilayerfilm, and a relationship of the complex amplitude of each electric field(incidence wave, transmitted wave, and reflection wave) before andbehind the interface is requested for each interface. The recurrenceformula is led from this relationship, and a relationship of the complexamplitude of the electric field of the incidence wave and reflectionwave of the multilayer film (in other words, complex reflectivity) isfinally calculated from start to the substrate side of the multilayerfilm. The phase is obtained from the imaginary part of the complexreflectivity.

The electric field strength of the standing wave of the surface isobtained from the result. The thickness of the silicon of the top layerof the multilayer film composed the molybdenum and silicon is assumed tobe a parameter in the calculated model. FIG. 13 shows the example of thecalculation result. The reflectivity and the electric field strengthratio of the surface respectively when the thickness of the silicon ofthe top layer of the multilayer film composed the molybdenum and siliconis 0, 2, 4, and 6 nm are plotted. The reflectivity hardly changes evenif the thickness of the silicon of the top layer changes. On the otherhand, the electric field strength ratio of the surface changesremarkably in proportion to the thickness of the silicon of the toplayer.

First, the fitting changes the coating cycle (sum of the thickness ofmolybdenum and silicon) of the multilayer film composed the molybdenumand silicon of the calculation model, the best coating cycle is obtainedso that the measurement value of the reflectivity must correspond to thecalculation value.

Next, the thickness of the silicon of the top layer of the calculationmodel is changed, and the best thickness of the silicon is obtained sothat the electric field strength ratio must correspond to themeasurement value. At this time, because an absolute value of theelectric field strength ratio has the uncertainty, the wavelengthdependency of electric field strength ratio corresponds. In other words,the constant and the thickness of the silicon of the top layer aredecided so that the product of the measured electric field strengthratio and the constant corresponds to the calculation value. Forexample, the square sum of the difference between the product of themeasured electric field strength ratio and the constant and the electricfield strength ratio obtained by the model calculation is assumed to bean evaluation function, the parameter is changed and fitting so that thevalue of this evaluation function becomes the minimum.

Thus, the calculation model that best reproduces the measurement valueis decided. Next, the phase of the reflected light of the multilayerfilm is obtained from the decided model. This phase is assumed to be themeasured phase difference between the incident light and the reflectedlight of the multilayer mirror ML. In the instant embodiment, the modelthat the thickness of the silicon of the top layer is 6 nm is the mostcorresponding to the measurement value shown in FIG. 12. The phaseobtained from this model is FIG. 14.

The phase of each point on the multilayer film is measured by using thismethod, and the shape of equivalently reflection surface for the EUVlight or the wave front of the reflected light when the EUV light isincident to this multilayer film can be obtained by adding themeasurement result of the multilayer mirror shape.

The instant embodiment can obtain the phase difference between theincident light and the reflected light even if the wavelength dependencyof the amount of the photoelectron discharged from the surface of thereference sample RS is not measured by comparing it with the modelcalculation, and the measurement is further simplified. Moreover, whenthe wavelength dependency of the amount of the photoelectron dischargedfrom the surface of the reference sample RS is measured, the phasedifference between the incident light and the reflected light may beobtained by comparing it with the model calculation. This method canobtain the phase difference between the incident light and the reflectedlight more precisely.

Third Embodiment

A description will be given of another more concretely method forobtaining “equivalent reflection surface” in the third embodiment.

In the instant embodiment, the EUV light is irradiated to the multilayermirror ML, and the reflectivity of the EUV light and amount of thedischarged photoelectron is measured at the same time. At this time, thereflectivity and the incidence angle dependency of the photoelectronamount are measured while changing the incidence angle of the EUV lightrelative to the sample. FIG. 15 shows an example of the measurementresult. The coating thickness of the multilayer film that addedmolybdenum to silicon is 8 nm, and the wavelength of the EUV light is13.5 nm. The phase difference is calculated by this result and Equation15.cos δ(λ)=(F(λ)−1−R(λ))/(2×R(λ)^(1/2))  (15)

To convert electric filed strength from the discharged photoelectricamount, the reference sample RS is used as well as the first embodiment,or the discharged photoelectron amount is standardized by the dischargeamount of the photoelectron at the incident angle with low reflectivityof the multilayer mirror ML, and the electric field strength ratio isobtained. The discharged amount of the photoelectron at the incidenceangle about 0° or 50° is used and standardized for this example.Commutatively, the discharged amount of the photoelectron of one photonis almost constant in the wavelength bands not close to the wavelengthof absorption edge of the material that composes the top layer of themultilayer film, so the wavelength is moved a little, and the dischargedphotoelectron amount may be standardized by the discharge amount of thephotoelectron measured by wavelength which greatly lowers thereflectivity of the multilayer film.

The phase of each point on the multilayer film is measured by using thismethod, and the shape of equivalently reflection surface for the EUVlight or the wave front of the reflected light when the EUV light isincident to this multilayer film can be obtained by adding themeasurement result of the multilayer mirror shape.

Fourth Embodiment

A description will be given of another more concretely method forobtaining “equivalent reflection surface” in the fourth embodiment.

Referring now to FIG. 16, a description will be given of a measuringapparatus 1A of the instant embodiment. Here, FIG. 16 is a schematicblock diagram of the measuring apparatus 1A of the instant embodiment.The measuring apparatus 1A of the instant embodiment is similar to thestructure of the measuring apparatus 1 shown in first embodiment.However, a measuring room 20A has been exhausted to the ultra-highvacuum by the exhaust part 21 such as the vacuum pump etc. to preventthe attenuation of the EUV light and the absorption of the fluorescentX-ray, or the pollution adhering to the multilayer film surface. Adetector 26A that detects the fluorescent X-ray is installed at near themultilayer mirror. The detector 26A uses a solid state detector (SSD),cooling SSD, and micro-calorimeter, etc. This detector may have acharacteristic in which energy of the photon of fluorescent X-ray isclassified. In other words, the detector measures the spectrum offluorescent X-ray, or only light intensity of fluorescent X-ray inspecific energy range.

A layer that consists of a specific material that differs from thematerial that composes the lower layer of the multilayer film is formedon the surface of the multilayer film sample. For example, the rutheniumlayer with the thickness of plural ones nanometer is installed in thesurface of the multilayer film that consists of the molybdenum andsilicon. The solid state detector is set to the detected energy range todetect only peculiar characteristic X-ray from an element that composesthe top layer.

The thin film that consists of ruthenium or carbon as a cap layer toprevent oxidation of the multilayer film and adhesion of contaminationis formed on the top layer of the multilayer mirror. Therefore, thedetected energy range is set to detect only the peculiar characteristicX-ray from the element that composes this layer.

The instant embodiment measures the phase of the reflected lightaccording to the process similar to the first embodiment. In this case,the first embodiment measures the amount of the photoelectron dischargedfrom the surface of the sample, but the instant embodiment measures thephase by measuring the amount of the fluorescent X-ray discharged fromthe surface of the sample. The solid state detector of the instantembodiment is set to the detected energy range to detect only peculiarcharacteristic X-ray to element that composes the top layer of themultilayer film, so the detected light intensity of the fluorescentX-ray is proportional to the electric field strength of the top surfaceof the multilayer film.

Fifth Embodiment

A description will be given of another more concretely method forobtaining “equivalent reflection surface” in the fifth embodiment.

The instant embodiment uses a similar measuring apparatus 1A shown inthe fourth embodiment. The instant embodiment measures the photoelectrondischarged from the multilayer film surface. The micro channel plate isused as a photoelectron detector as shown in FIG. 5. Here, FIG. 5 is atypical view for measuring the electric field strength ratio of themultilayer film surface. The EUV light is irradiated to the multilayerfilm sample, and the photoelectron discharged by the photoelectriceffect is incidence to the MCP. To efficiently collect thephotoelectrons, the voltage is applied to the surface of the incidenceside of MCP to be the potential of the plus for the multilayer film (forinstance, potential from about plus 100 to 500 volts).

A strong potential difference from about 2000 to 6000 volts is appliedin the MCP for electron acceleration, amplifies an incidence electronfrom about 106 to 108, and is discharged from the exit side. Thiselectron collides to a fluorescence board, maintained at a higherpotential higher than the MCP exit side, and generates the fluorescenceof visible light. This fluorescence is detected by the photodetector,for example, photodiode and photoelectron multiplier. The MCP exit sideis maintained at a high positive voltage, and the fluorescence board ismaintained at a higher positive voltage (for instance, about plus 3000to 8000 volts to the multilayer film) to attract the electron. However,because the electron is converted into visible light by the fluorescenceboard, the photodetector can be set to an arbitrary potential. Forexample, the photodetector is maintained at the same potential as themultilayer film.

When the electron is detected while amplified, the output of thedetector becomes a high voltage of the plus. Therefore, the techniquefor cutting the direct current at the condenser and inputting the onlyalternating current element that changes timewise is used to input it tothe signal processing system. This method is effective to the pulselight source such as the laser plasma and the discharge plasma lightsource, etc. that changes timewise. However, when the consecutivetimewise light such as the synchrotron radiation (SR) is used for thelight source, this method to which direct current element is interceptedby the condenser cannot be used. When the fluorescence that generates byirradiating the electron output from MCP to the fluorescence board isdetected by the photodetector, the photodetector can be maintained thesame potential as the multilayer film. Therefore, there is an advantagethat can be directly input to the signal processing system.

The instant embodiment measures the phase of the reflected light byusing the measuring apparatus 1A according to the process similar to thesecond embodiment. Therefore, the detail is omitted.

Sixth Embodiment

A description will be given of another more concretely method forobtaining “equivalent reflection surface” in the sixth embodiment.

The instant embodiment irradiates the EUV light to the multilayer mirrorML, and measures the reflectivity of the EUV light and the amount of thefluorescent X-ray at the same time according to the process similar tothe third embodiment. In this case, the third embodiment measures theamount of the photoelectron discharged from the surface of the sample,but the instant embodiment measures the phase by measuring the amount ofthe fluorescent X-ray discharged from the surface of the sample.

Seventh Embodiment

A description will be given of more concretely correcting method of themultilayer film in the seventh embodiment.

The instant embodiment executes the coating milling as a correction ofthe multilayer film.

The coating milling has been known as a method for correcting a surfaceshape of a plate in each multilayer mirror as proposed in “SUB-nm,Figure Error Correction of a Multilayer Mirror by Its Surface Milling”,Masaki Yamamoto, Nuclear Instruments and Method in Physics Research A.,467-468 (2001), pp. 1282-1285. A description will be given of thecoating milling with reference to FIGS. 17 to 22.

As shown in FIG. 17A, parallel light with an equal phase incident upon amultilayer mirror that uniformly forms a multilayer film on a mirrorsubstrate would provide reflected light having a completely equal phaseor reflected wave front as shown in FIG. 17B. On the other hand, asshown in FIG. 18A, wave front of the reflected light from part, at whichone pair of films form unevenness on the multilayer film, forms a phasedifference as shown in FIG. 18B.

The reflectance of the multilayer mirror depends upon the number oflayer pairs of the multilayer film. FIG. 19 shows the number of layerpairs of the multilayer film and the reflectivity standardized by themaximum value. The reflectivity increases with the increased periodicityup to the forty layer pairs, and saturates above the forty layer pairs.If a multilayer film has been layered with sufficient periodicity, e.g.,sixty layer pairs after the reflectance saturates, a difference inperiodicity in the multilayer film would affect only a wave front.

It explains the case to incident the EUV light of 13.5 nm to a Mo/Simultilayer mirror that is formed by alternately forming or layeringmolybdenum and silicon at the incidence angle of 10°. The thickness ofthe Mo layer is about 3 nm, the thickness of the Si layer is about 4 nm,and the coating cycle is about 7 nm.

Suppose that the multilayer film is removed from its top layer while thetop layer is set to be an origin in the multilayer film. The removalamount from the multilayer film is called a milling depth. FIG. 20A is agraph of a relationship between the milling depth and reflectivity,while FIG. 20B shows a graph between the milling depth and the wavefront shift amount, when the EUV light of 13.5 nm is incident at anangle of 10° upon a Mo/Si multilayer mirror. In general, a Mo/Simultilayer arranges the Si layer as the top layer to reduce theinfluence of the oxidization of Mo, and the instant embodiment hascalculated on the assumption that the Si layer is located as the toplayer. It is understood from FIGS. 20A and 20B that as one layer pair of6.99 nm is removed from the multilayer film, the wave front of thereflected light moves by about 0.025 wavelength. FIG. 20C shows a graphthat converts a shift amount of wave front into a shift amount of aspatial reflection position. The shift amount L of a spatial reflectionposition is given in λ×W=2L, where λ is a wave front of incident lightand W is a shift amount of wave front. A removal of one layer pair of6.99 nm from the multilayer film means about 0.2 nm movement of thereflected position in the instant embodiment. As understood from FIG.20A, the coating milling changes the index and wave front in the Molayer more greatly than those in the Si layer due to a relationship ofreflectivity. As above-mentioned, as the reflectivity saturates for theperiodicity of a multilayer film that has about sixty layer pairs, aremoval of one layer pair would change the wave front without changingthe reflectivity.

Use of a relationship described with reference to FIGS. 17 to 20 wouldeasily correct about 0.2 nm by a removal of one layer pair of 6.99 nmfrom the multilayer film. This is called the coating milling.

For example, suppose as shown in FIG. 21A, a multilayer mirror thatuniformly forms a multilayer film on a distorted mirror substrate. Thecoating milling is an approach to delay a phase, and applied to a pointA as an origin having the latest phase. The Si layer changes the wavefront slightly, whereas the Mo layer changes the wave front greatly and,as above-mentioned, is highly vulnerable to oxidization. Therefore,without special coating, it is not desirable to finish coating millingin the middle of the Mo layer and continuously adjust the wave front. Asshown in FIG. 21B, the wave front is adjusted discontinuously byremoving every layer pair of Mo and Si. On the other hand, since the Silayer has resistance to oxidation and does not affect the wave front,the coating milling may end in the middle of the Si layer. Asabove-mentioned, when the EUV light of 13.5 nm is incident at an angleof 10°, a removal of one layer pair of 6.99 nm from the multilayer filmmay easily correct the spatial reflection position or a figure error ofthe mirror substrate every 0.2 nm.

Referring to FIGS. 21A and 21B, when a surface shape of the mirrorsubstrate has a figure error of 0.4 nm at a point B and a figure errorof 0.2 nm at a point C viewed from the point A, the wave frontaberration caused by the figure error of the mirror substrate may becorrected by removing two layer pairs from the multilayer film at thepoint B and one layer pair from the multilayer film at the point C.

Similarly, as shown in FIG. 22A, for example, suppose a multilayermirror that uniformly forms a multilayer film on a mirror substrate thathas a point F at a center part above a point E on the edge. Since thepoint E has relatively the latest phase in the multilayer mirror, thecoating milling is applied to a point E as an origin. Referring to FIGS.22A and 22B, when the mirror substrate has a figure error of about 0.4nm between the edge point E and the center point F, and the figure errorchanges continuously, two layer pairs are removed from the multilayerfilm at the center point F. The wave front aberration caused by thefigure error may be corrected by removing one layer pair at both sides.

The instant embodiment explained the correcting method of the multilayerfilm when the substrate has the figure error. However, this method issimilarly applicable when the “equivalent reflection surface” has thefigure error.

When the error from the design value of the EUV reflection wave frontcalculated in step 1006 of FIG. 3 is outside the tolerance, themultilayer film correcting method of the instant embodiment is used todecrease the error. Therefore, an optical element that has a multilayerfilm with a desired performance can be easily fabricated.

There is an ion beam etching apparatus as an apparatus that can applythe coating milling and remove thin film. A description will be given ofthe ion beam etching apparatus. The ion beam apparatus irradiates theion beam accelerated at a plasma generation room to the mirror, andetches a target etching film (multilayer film). Because the ion beametching can be controlled comparatively easily because it is controlledelectrically. Moreover, the ion beam etching is a dry process, pollutesvery little, and influences other mirror parts very little.Directionality is good, and the partial etching is possible by forming ashield part using a masking plate.

Moreover, if it is the dry process of a sputtering etching etc., besidesthe ion beam etching, the etching of the multilayer film is possible.

A correcting apparatus that corrects the optical element having themultilayer film can be composed by combining the above measuringapparatus with the etching apparatus. The correcting apparatus canexecute the EUV wave front measurement and the correcting method of theinstant embodiment, and facilitates the correction of the opticalelement that has the multilayer film.

Eighth Embodiment

Next, a description will be given of other correcting method of themultilayer film.

The instant embodiment uses a method of adding the film to themultilayer film as the correcting method of the multilayer film.

The shift W of the reflection phase of EUV light for each unit lengthand the shift L of the reflection phase of EUV light that moves byadding the thickness d are given in Equations 16 and 17 respectively,where the wavelength of the incident light is λ, the index is n, and theadding thickness is d.W=(4π/λ)×(1/n)  (16)L[radian]=(4π/λ)×(1/n)d  (17)

The best thickness of an addition film is decided based on Equations 16and 17. For instance, ruthenium is used as a material of the additionfilm.

When the error from the design value of the EUV reflection wave frontcalculated in step 1006 of FIG. 3 is outside the tolerance, themultilayer film correcting method of the instant embodiment is used todecrease the error. Therefore, an optical element that has a multilayerfilm with a desired performance can be easily fabricated.

A correcting apparatus that corrects the optical element having themultilayer film can be composed by combining the above measuringapparatus with the deposition apparatus. The correcting apparatus canexecute the EUV wave front measurement and the correcting method of theinstant embodiment, and facilitates the correction of the opticalelement that has the multilayer film.

Ninth Embodiment

Next, a description will be given of another correcting method of themultilayer film.

The instant embodiment uses a phase correcting method that changes afilm structure by heating the film with the electron beam as a method ofcorrecting the multilayer film.

An electron beam irradiating apparatus is used as a heat source. Thebeam with a predetermined energy is irradiated to the corrected place onthe reflection wave front, the film structure is changed, and the phaseis corrected.

When the error from the design value of the EUV reflection wave frontcalculated in step 1006 of FIG. 3 is outside the tolerance, themultilayer film correcting method of the instant embodiment is used todecrease the error. Therefore, an optical element that has a multilayerfilm with a desired performance can be easily fabricated.

A correcting apparatus that corrects the optical element having themultilayer film can be composed by combining the above measuringapparatus with the electron beam irradiating apparatus. The correctingapparatus can execute the EUV wave front measurement and the correctingmethod of the instant embodiment, and facilitates the correction of theoptical element that has the multilayer film.

Tenth Embodiment

Next, a description will be given of another correcting method of themultilayer film.

The instant embodiment uses a method that gives a deformation by addinga power from the outside to the multilayer mirror substrate. Therefore,the instant embodiment adds the power to the multilayer mirror substrateand deforms the substrate, consequently, corrects the multilayer film bydeforming the multilayer film.

FIG. 23 is a view of a situation to correct a deformation of thesubstrate by an actuator and above coat milling. A multilayer film 102is formed to a substrate 101, and an actuator 104 that transforms thesubstrate is arranged between the substrate 101 and a high rigiditymember 103. The member 103 and the actuator 104 are installed before orafter the multilayer film is formed.

For instance, a piezo-electric element is suitable as the actuator 104.The piezo-electric element can control minute displacement, and canenlarge the displacement amount by piling up like the stack. Amicro-moving mechanism may be composed when the piezo-electric elementand a hinge spring are combined.

The number of piezo elements changes in the transformed degree etc. Inthe instant embodiment, four piezo-electric elements are arranged at asection position of the direction of the diameter to correct thedeformation of degree as size of the mirror with the same cycle.Actually, the piezo-electric elements are arranged along the entirecurved surface, but figure is omitted.

The member 103 is installed in consideration of the transformation whenthe member is installed. The surface is polished with the voltage notadded to the piezo-electric element, and is polished until the errorfrom the designed surface becomes about 5 nm. The polishing with theaccuracy of the error of about 5 nm can be achieved comparativelyeasily.

When the error from the design value of the EUV reflection wave frontcalculated in step 1006 of FIG. 3 is outside the tolerance, themultilayer film correcting method of the instant embodiment is used todecrease the error. Therefore, an optical element that has a multilayerfilm with a desired performance can be easily fabricated.

Embodiment of the Exposure Apparatus

Referring now to FIG. 2, a description will be given of a EUV exposureapparatus 100 of the present invention. FIG. 2. is a schematic sectionalview of the EUV exposure apparatus. The EUV exposure apparatus 100 is aprojection exposure apparatus that uses, as illumination light forexposure, EUV light (e.g., with a wavelength of 13.5 nm) to perform astep-and-scan exposure that transfers a circuit pattern on a reticleonto an object to be exposed. Referring to FIG. 2, the EUV exposureapparatus 100 includes a EUV light source part 110, an illuminationoptical system 120, a reflection-type mask (reflection-type reticle)130, a mask stage 132, a projection optical system 140, a wafer 150, anda wafer stage 152. A vacuum chamber VC2 houses the illumination opticalsystem 120 to the wafer stage 152.

The EUV light source part 110 irradiates a highly intensified pulselaser light PL to a target material supplied from a target supply system112 arranged in a vacuum chamber VC1 and put in a condenser pointposition 113, via a condenser optical system (not shown) from a laserlight source (not shown), thus generating high-temperature plasma foruses as EUV light with a wavelength of about 13.5 nm emitted from this.The EUV light source part 110 excites the target material tohigh-temperature plasma by irradiating a high-luminance excitation pulselaser to the target material, corrects the EUV light from the light witha wavelength band from infrared, ultraviolet to EUV light that isisotropically irradiated from the plasma when the plasma is cooled by acondenser mirror 114, and uses this as the exposure light.

The pulse laser PL uses, for example, Nd:YAG laser or excimer laser. Thevacuum chamber VC1 maintains a vacuum atmosphere environment for the EUVlight with a small transmittance to the atmosphere is small. The pulselaser is condensed in the condenser point position 113 through a window111 installed in the vacuum chamber VC1. The target material depends onthe wavelength of the generated EUV light, uses a metallic thin filmsuch as copper (Cu), lithium (Li), and zinc (Zn) etc., an inert gas suchas xenon (Xe) etc., and a liquid drop, etc., and is supplied to thevacuum chamber VC1 by the target supply system 112 such as a gas jet.The target supply system 112 has a target recover system that recovers aremained target material because all of the supplied target materialdoes not contribute to the plasma.

The EUV light introduced into the vacuum chamber VC2 illuminates themask 130 that has a predetermined pattern through the illuminationoptical system 120. The illumination optical system 120 leads the EUVlight, and illuminates the mask 130. The illumination optical system 120includes a plural mirror, an optical integrator, and an aperture. Theoptical integrator serves to uniformly illuminate the mask with apredetermined NA. The aperture is arranged in a conjugate position forthe mask 130, limits an illumination area to an arc shape on the mask130.

The EUV light selectively reflected by the reflection-type mask 130 isprojected onto the wafer 150 that a photoresist applied by theprojection optical system 140 composed of the plural reflection mirror,and transfers the pattern of the mask 130 to the wafer 150.

The illumination area of the mask 130 and a projection image of thewafer 150 are limited within an arc shape area of extremely narrow sameimage height to obtain an excellent image that suppresses aberration ofthe projection optical system 140. Then, the exposure apparatus 100adopts the scanning exposure method that exposes by synchronouslyscanning the mask stage 132 and the wafer 152 to expose all patternsformed in the mask 130 to the wafer 150.

The condenser mirror 112, the illumination optical system 120, thereflection-type mask 130, and the projection optical system 140 have themultilayer film of Mo and Si on the substrate to efficiently reflect theEUV light, and the surface roughness requires 0.1 nm on standarddeviation to suppress a decrease of the reflectivity. Moreover, thereflection mirror of the projection optical system requires the shapeprecision of 0.1 nm on standard deviation in addition to above surfaceroughness, and needs an extremely high precision optical system. Theoptical element fabricated by the fabrication method of the presentinvention is applied to such the optical element. Therefore, theexposure apparatus of the instant embodiment can be exposed in highaccuracy.

Embodiment of the Device Fabrication Method

Referring now to FIGS. 24 and 25, a description will be given of anembodiment of a device fabrication method using the above mentionedexposure apparatus 100. FIG. 24 is a flowchart for explaining how tofabricate devices (i.e., semiconductor chips such as IC and LSI, LCDs,CCDs, and the like). Here, a description will be given of thefabrication of a semiconductor chip as an example. Step 1 (circuitdesign) designs a semiconductor device circuit. Step 2 (maskfabrication) forms a mask having a designed circuit pattern. Step 3(wafer making) manufactures a wafer using materials such as silicon.Step 4 (wafer process), which is also referred to as a pretreatment,forms the actual circuitry on the wafer through lithography using themask and wafer. Step 5 (assembly), which is also referred to as apost-treatment, forms into a semiconductor chip the wafer formed in Step4 and includes an assembly step (e.g., dicing, bonding), a packagingstep (chip sealing), and the like. Step 6 (inspection) performs varioustests on the semiconductor device made in Step 5, such as a validitytest and a durability test. Through these steps, a semiconductor deviceis finished and shipped (Step 7).

FIG. 25 is a detailed flowchart of the wafer process in Step 4. Step 11(oxidation) oxidizes the wafer's surface. Step 12 (CVD) forms aninsulating layer on the wafer's surface. Step 13 (electrode formation)forms electrodes on the wafer by vapor disposition and the like. Step 14(ion implantation) implants ion into the wafer. Step 15 (resist process)applies a photosensitive material onto the wafer. Step 16 (exposure)uses the exposure apparatus 100 to expose a circuit pattern from themask onto the wafer. Step 17 (development) develops the exposed wafer.Step 18 (etching) etches parts other than a developed resist image. Step19 (resist stripping) removes unused resist after etching. These stepsare repeated to form multi-layer circuit patterns on the wafer. Use ofthe fabrication method in this embodiment helps fabricate higher-qualitydevices than conventional methods. Thus, the device fabrication methodusing the exposure apparatus 100, and resultant devices constitute oneaspect of the present invention.

Furthermore, the present invention is not limited to these preferredembodiments and various variations and modifications may be made withoutdeparting from the scope of the present invention.

Thus, a fabrication method of optical element of the present inventioncan easily fabricate an optical element that has a multilayer film witha desired performance.

This application claims foreign priority benefits based on JapanesePatent Applications No. 2004-139060, filed on May 7, 2004, which ishereby incorporated by reference herein in its entirety as if fully setforth herein.

1. An evaluation method of an optical element having a multilayer filmcomprising the steps of: measuring a secondary radiation radiated fromthe multilayer film when a light with a wavelength of 2 to 40 nm isirradiated to the multilayer film, determining a phase differencebetween the light irradiated to the multilayer film and the lightreflected from the multilayer film based on a measurement result of themeasuring step; and judging whether to use the optical element based onthe determined phase difference.